I replaced the rear pads on a 2009 Sonata GLS 4 cyl with 32.5K miles.
The passenger rear caliper break piston boot was had seperated and caused the caliper to stick. It also was seperating on the driver side too BUT I cought that one in time.

I had to move the piston in and out and clean it up. It had some rust in it. After three time is moved a lot easier.
The trick to putting the boot back in is to compress the piston in as far as it will go then use a small screw driver to push the boot in all the way around. A little oil also helps. If all goes right it should be able to twist around to help get it in all the way around.

See photo of the drivers side. Good luck.

 

Still failing. Going the dealer and see what they will/will not do for me.

 

From the appearance of the pix there is a lot of rust and corrosion that should be a clue to replace that/ those calipers. There is a square "O" ring around the caliper piston and rust will score that ring and produce a brake fluid leak and just plain brake problems.
It's not worth the gamble to try to "clean up" the piston. You want the brakes to be there when you step on that pedal.
Usually easier and safer to replace the calipers with good rebuilt units.